As a conventional color image forming apparatus, for example, a so-called tandem type apparatus is known in which a plurality of image forming units are arranged in order with respect to a paper-conveying path along a horizontal direction, and toner images are transferred sequentially from the image forming units onto a paper sheet being conveyed along the paper-conveying path so that a color image is formed on the paper sheet. As an optical scanning device used in the tandem type color image forming apparatus, a device merely using four optical scanners with each scanning a single light beam (see JP2000-141759 A), a device using a single optical deflector and four sets of lens systems (see JP2001-133717 A), or a device using four sets of curved surface mirrors and lenses (see JP10(1998)-148777 A) is known.
However, the optical scanning devices proposed as above have presented the following problems. That is, the optical scanning devices require a large number of components and thus are costly. Further, in the optical scanners, uniform performance of scanning lines hardly can be provided.